Not Applicable
The present invention relates generally to a method and apparatus for directing or focusing dispersed charged particles into a low pressure apparatus. More specifically, the invention utilizes a multi-capillary inlet to increase the conductance of ions and other charged particles generated at or near atmospheric pressure into a relatively low pressure region, which allows increased efficiency in transmitting those ions and other charged particles.
A great variety of scientific inquiry is confronted with the challenge of identifying the atomic structure or composition of particular substances. To assist in this identification, a variety of schemes have arisen which require the ionization of the particular substances of interest. Many of these analytical techniques, as well as the other industrial uses of charged particles, are carried out under conditions of high vacuum. However, many ion sources operate at or near atmospheric pressures. Thus, those skilled in the art are continually confronted with challenges associated with transporting ions and other charged particles generated at atmospheric or near atmospheric pressures into regions maintained under high vacuum.
An illustrative example of this general problem is presented in the use of electrospray ionization when combined with mass spectrometry as an analytical technique. Electrospray ion sources (which broadly includes, but is not limited to, nano electrosprays, conventional electrosprays, micro-electrospray, and nebulizing gas assisted electrospray) are widely used with mass spectrometry for sample analysis, for example in biological research. For m/z analysis, ions are typically created at atmospheric pressure by the electrospray ion source and are then transported to the high vacuum region of a mass spectrometer through a capillary inlet that penetrates the first chamber of the mass spectrometer. A differential pumping system involving several stages for stepwise pressure reduction is commonly used to achieve the vacuum conditions conventionally utilized in m/z analysis within the mass spectrometer, and the major design issues are generally related to optimizing overall ion transmission efficiencies.
Improved transmission efficiencies in the intermediate vacuum stages have been achieved by using the recently developed RF ion funnel at higher interface pressures (xcx9c1 to 10 Torr) and RF multi-pole ion guides with buffer gas cooling at lower interface pressures as more fully described in Shaffer, S. A.; Tang, K.; Anderson, G. A.; Prior, D. C.; Udseth, H. R.; Smith, R. D., Rapid Commun. Mass Spectrom. 1997, 11, 1813-1817; Shaffer, S. A.; Prior, D. C.; Anderson, G. A.; Udseth, H. R. and Smith, R. D. Anal. Chem. 1998, 70, 4111-4119; and Douglas, D. J.; French, J. B., J. Am. Soc. Mass Spectrom. 1992, 3, 398-408, and U.S. Pat. No. 6,107,628 entitled Method and Apparatus for Directing Ions and other Charged Particles Generated at Near Atmospheric Pressures into a Region under Vacuum, the entire contents of each of which are herein incorporated into this specification by this reference.
However, in the region where the ions of interest are generated, the total charge transmission is directly proportional to the cross section area of the inlet orifice diameter or capillary inner diameter. To improve the ion transmission in this region, a larger inlet is clearly desired, but the inlet size is limited by several factors. For example, simply using a larger inside diameter (I.D.) capillary inlet is problematic. First, the desolvation is less effective for larger I.D. capillary inlets because of the greater temperature variation across the capillary radius (resulting in a large variation in droplet desolvation efficiency). A second problem is the ion transmission efficiency in the first vacuum stage may be decreased due to greater gas dynamic effects. Thus, there is still a general need for improved methods for generating ions at atmospheric pressures, and a particular need for an efficient ion transmission while maintaining the effective droplet desolvation for the ion currents relevant to electrospray ionization (ESI) where aerodynamic effects dominate. Ion transmission between an ion source and the first vacuum stage is primarily dependent upon the proximity and gas conductance of the interface inlet.
Accordingly, it is an object of the invention in one of its aspects to provide a method for providing an ion or charged particle source in a pressure region at near atmospheric pressures. As used herein, xe2x80x9cnear atmosphericxe2x80x9d pressures are defined as between 10xe2x88x921 millibar and 1 bar. Also as used herein, the charged particles are defined as being smaller than one billion AMUs. The focusing of the present invention is accomplished by providing an apparatus, hereinafter referred to as a xe2x80x9cmulti-capillary inletxe2x80x9d, which is operated at the interface between an ESI source and the interior of an instrument maintained at near atmospheric pressures. To demonstrate a preferred embodiment of the present invention, a prototype multi-capillary inlet was constructed from an array of seven thin wall stainless steel tubes soldered into a central hole of a cylindrical heating block. However, those skilled in the art will recognize that the advantages of the present invention may be achieved by providing a plurality of narrow passageways or orifices through which a flow of charged particles may be directed, regardless of the particular method of fabrication. While interfaces formed of capillaries as described herein are the preferred method of fabrication, interfaces having essentially equivalent physical dimensions can be fabricated by a variety of means well known to those having skill in the art, and the use of the term xe2x80x9cmulti-capillaryxe2x80x9d should not, therefore, be construed to limit the scope of the invention. Rather, the present invention should be construed as including any apparatus whereby a plurality of passageways are formed as the interface between an ion source, such as an ESI, and the interior of an instrument maintained at near atmospheric pressure. These would include, but not limited to, an interface formed by drilling a plurality of passageways into a block of material, an interface formed by casting a block of material with passageways formed in a casting process or molding process, and an interface formed by providing an array of capillaries as described in the description of the preferred embodiment herein.
xe2x80x9cWhile the present invention should be broadly construed to include any application wherein the multi-capillary inlet is desired juxtaposed between an ion source and the interior of an instrument maintained at near atmospheric pressure, it finds particular advantages when deployed to improve the ion transmission between an ESI source and the first vacuum stage of a mass spectrometer, and finds its greatest advantages when deployed in conjunction with an electrodynamic (RE) ion funnel deployed within the interior of the mass spectrometer. When deployed in this fashion, the multi-capillary inlet described herein has been demonstrated to provide more uniform droplet evaporation conditions than are provided by a single capillary having the same gas conductance. The present invention is further advantageously deployed with an ion funnel equipped with a jet disturber, as described in U.S. Pat. No. 6,583,408, filed May 18, 2001, xe2x80x9cImproved Ionization Source Utilizing a Jet Disturber in Combination with an Ion Funnel and Method of Operationxe2x80x9d the entire contents of which are incorporated herein by this reference.xe2x80x9d
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.